A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes), which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea. In such cases a cochlear implant is an auditory prosthesis which uses an implanted stimulation electrode to bypass the acoustic transducing mechanism of the ear and instead stimulate auditory nerve tissue directly with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processing stage 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant stimulator 108. Besides extracting the audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an implanted electrode carrier 110. Typically, this electrode carrier 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
In some persons, the cochlear shape fails to develop properly and various malformation conditions can occur such as those shown in FIG. 2: cochlear aplasia, cochlear hypoplasia, common cavity (CC) malformation, and incomplete partitioning. Specifically in a common cavity malformation the cochlea and the vestibule are represented by a single chamber. This structure may have cochlear and vestibular neural structures, but it completely lacks inter-scala separation (no basilar membrane), no modiolus trunk, and it appears as a single cavity. The neural structures are believed to be present at the bony capsule defining the outer cavity wall. The specific size of the cavity can vary significantly and can be measured using medical imaging.
Placing an electrode inside a malformed common cavity cochlea is not straightforward and needs utmost care to ensure that the stimulation contacts are either touching or very close to the outer wall of the cavity. The current technique involves making two cochleostomy openings in the outer surface of the cochlea for the electrode placement, which is undesirably traumatic.
FIG. 3A shows one approach wherein the electrode array 302 has an extended distal end. Two cochleostomies 304 are made in the outer surface of the cochlea 300, the electrode array 302 is inserted through one of the cochleostomies 304, and the distal tip of the electrode array 302 is retrieved and pulled through the other cochleostomy 304. The surgeon has to manipulate the electrode array 302 to attempt to place the stimulation contacts 303 against the outer wall 301 of the cavity, after which the final position of the electrode array 302 is fixed and the distal extension may be removed.
FIG. 3B shows another approach for electrode implantation in a common cavity, again requiring two cochleostomies 304 in the outer surface of the cochlea 300. Two separate electrode arrays 302 are used, one through each cochleostomy 304, and again considerable surgical skill is needed to manipulate the electrode arrays 302 to place their stimulation contacts 303 adjacent to the outer wall 301 of the cavity. Both techniques are highly traumatic in requiring two cochleostomies and both require considerable surgical skill to be effective.